1. Field of the Invention
The present invention relates to a receiver such as a wireless communication apparatus, for example, including a low-noise amplifier which can be applied to a television tuner or the like.
2. Description of the Related Art
A low-noise amplifier is used in a wireless receiver typified by a television tuner.
Japanese Patent Laid-Open. No. 2008-17058 discloses a strain compensating circuit used for linearization of a low-noise amplifier or the like.
FIG. 1 is a circuit diagram showing an example of a configuration with which a differential amplifier composing a low-noise amplifier, and a strain compensating function are both realized.
As shown in FIG. 1, the low-noise amplifier 1 includes n-channel MOS (NMOS) transistors NT1 and NT2 composing a differential pair transistor 11, and a current source 12.
A strain compensator 2 includes NMOS transistors NT3 and NT4 composing a differential pair transistor 21, and a current source 22.
Hereinafter, the principles of an operation of the circuit shown in FIG. 1 will be described.
In the differential pair composed of two MOS transistors each as an insulated gate field-effect transistor conforming to square characteristics, under a constant input amplitude, a relationship among an amplitude IM3 of a three-order strain component, an over-drive voltage (Vgs−Vth), and a transistor size, W, is expressed by Expression (1):
                              IM          ⁢                                          ⁢          3                ∝                  α          ⁢                                          ⁢                      W                          (                              Vgs                -                Vth                            )                                                          (        1        )            
where α is a proportionality coefficient peculiar to a device process.
With regard to a strain compensation establishment condition, the amplitude IM3 of the three-order strain component generated by the strain compensator 2 is identical to that IM3 of the three-order component generated by the low-noise amplifier 1. Therefore, when a ratio of a transistor size of the strain compensator 2 to a transistor size of the low-noise amplifier 1 is set at 1:M from Expression (1), the following relational expression (Expression (2)) is established between the current ratios:Ic=Id×M3  (2)
where Id is a consumption current in the compensating circuit, and Ic is a consumption current in the low-noise amplifier.
Also, a total gain in the compensation establishment state is reduced as expressed by Expression (3) in terms of a circuit configuration:
                    G        ⁡                  (                      1            -                                          1                                  M                  5                                                              )                                    (        3        )            
where G is a gain in a phase of ratio compensation.